The substantial and continuous efforts to recover hydrocarbon fluids from underground reservoirs has brought on the realization that subterranean earth formation damage, which reduces hydrocarbon fluid recovery, can occur through the use of conventional liquid drilling fluids, such as so-called drilling muds. These fluids, which usually comprise water or refined hydrocarbon liquids, a weighting agent, viscosifiers and lost circulation prevention substances, can invade the formation from the wellbore while circulating the fluids during the drilling process and resulting in damage to the formation with respect to efforts to recover hydrocarbon fluids therefrom. Penetration of drilling fluids into the formation occurs, of course, when the pressure forces of the fluids in the well exceed the natural formation pressure. However, conventional drilling techniques include maintaining a so-called overbalanced or net positive pressure of the drilling fluid over and above the formation pressure to minimize contamination of the drilling fluid with formation fluids and to minimize the chance of well blowout.
Efforts to overcome the potential for damage created by drilling with conventional liquid drilling fluids or muds in overbalanced conditions have resulted in the development of so called underbalanced drilling techniques wherein the hydrostatic pressure of the drilling fluid in the well is maintained at a value less than the formation pressure to minimize penetration of the drilling fluids into the formation from the wellbore wall interface. Still further, where formation conditions permit, drilling operations have been carried out with compressed air, natural gas and other gasses as the drilling fluid. When environmental and economic conditions permitted the use of natural gas as a drilling fluid in a so-called open circulation system, this technique was widely used. However, the commercial value of natural gas and environmental considerations have resulted in substantial elimination of drilling operations wherein natural gas is used as the circulation fluid but is vented to atmosphere or "flared" after returning from the borehole with entrained drill cuttings.
Drilling with compressed air as the cuttings evacuation fluid also tends to oxidize formation fluids in situ and raise the hazard of ignition of formation produced combustible gasses, such as natural gas, when mixed with the compressed air in the circulation system. Moreover, heretofore, other problems associated with operating a closed gas circulation system for well drilling have prevented use of these systems with inert gas or compressed air.
Use of natural gas as the cuttings evacuation fluid, in particular, in a well drilling system, has certain advantages in underbalanced operating conditions. Natural gas is often in plentiful supply in hydrocarbon reservoirs and nearby formations and may be a product of the reservoir itself in many formations. The use of natural gas as a drilling fluid reduces the hazards of operating in an overbalanced condition because the gas minimizes formation damage in liquid hydrocarbon as well as hydrocarbon gas producing or storage reservoirs and, in fact, can enhance formation productivity through its miscibility with formation liquids and its effectiveness as a drive fluid.
Moreover, drilling operations carried out in so called underbalanced or substantially underbalanced pressure conditions in the wellbore can possibly bring about the realization of as much as a 10-fold increase in the rate of penetration in geo pressured reservoirs and hard rock formations such as hard sand, dolomite and limestones. This increase in the rate of penetration is accomplished due to the fact that earth formations are much weaker in tension than in compression. Accordingly, by reducing wellbore pressures which would place the formation in compression at the point of penetration of the formation these dramatic increases in the rates of penetration may be realized, particularly with a closed gas drilling fluid circulation system.
However, a closed gas circulation system presents certain problems, including drill cuttings separation and sampling from the gas circulation system, treatment of the gas so that it is suitable for recirculation through the drill string and the wellbore or discharge to a gas transport pipeline, and well control to prevent unwanted blowouts or fire resulting from the presence of a combustible fluid. These problems have been substantially overcome by the present invention as will be appreciated by those skilled in the art from reading the following summary and a detailed description of the system, its components and methods of operation in accordance with the invention.